In radio frequency (RF) communications, it is inevitable that undesired frequency signals reach a receiver's RF mixer, mixed into desired signal bandwidth, and cause receiver sensitivity degradation. For example, in a radio base station, a 768 MHz sinusoidal clock signal may be fed to a transmitter DAC (digital to analog converter) where it is divided by 2 and used to generate a square wave clock signal at 384 MHz. If the receiver is tuned to exactly 3.456 GHz, the harmonic of ninth order of the square wave, which is also a sinusoid at 3.456 GHz, radiates from the digital section to the receiver RF front end circuitry that resides in the same board and gets mixed down to the receiver intermediate frequency (IF) (e.g., 468 MHz) after going through the receiver mixer. In the output of a Fast Fourier transform (FFT) of the baseband signal, this harmonic component, which is called a self-quieter, will peak in the frequency domain and may wipe out a number of tones in Orthogonal frequency division multiplexing (OFDM) systems. FIG. 1 is a graph that shows a self-quieter observed in an OFDM base station receiver. Graph 100 depicts RSSI receive signal strength indicator (RSSI) in dB verse tone index. Some hardware methods such as adding extra shielding or foam may mitigate this self-quieter problem. However, they are too expensive for ordinary implementations.
Some other relevant techniques for addressing the self-quieter problem are known in the literature. Examples of these include: Gaikwad, Rohit V. and Trachewsky, Jason A., “Spur harmonic canceller for RF band clock”, EP1648093 A1, Broadcom Corporation, 2005, (See also in US Patent Application US 20060093019A1); Choi, Won-Joon, Gilbert, Jeffrey, M., Wang, Yi-Hsiu and Zhang, Xiaoru, “Spur Mitigation Techniques”, US20050059366A1, Atheros Communications, Inc., 2005; and J. I. Danzig, K. L. Miller and H. R. Whitehead, “System and Method for Spur Cancellation”, US20060087461A1, Broadcom Corporation, 2005. Such techniques utilize frequency mixing and notching filters in order to perform time-domain spur cancellation. However, for various broadband OFDM systems like WiMax, 3GPP (3rd Generation Partnership Project) Long Term Evolution (LTE) and those of 3GPP2 (3rd Generation Partnership Project 2), time-domain spur cancellation requires intensive computation that is proportional to the sampling rate. (The standards groups corresponding the systems mentioned may be contacted via http://www.wimaxforum.org/, http://www.3gpp.org/, and http://www.3gpp2.com/, respectively.) Thus, there is a need for additional cancellation techniques to address the self-quieter problem, which are more practical for use in OFDM systems.
Specific embodiments of the present invention are disclosed below with reference to FIGS. 2-6. Both the description and the illustrations have been drafted with the intent to enhance understanding. For example, the dimensions of some of the figure elements may be exaggerated relative to other elements, and well-known elements that are beneficial or even necessary to a commercially successful implementation may not be depicted so that a less obstructed and a more clear presentation of embodiments may be achieved. In addition, although the signaling flow diagrams and/or the logic flow diagrams above are described and shown with reference to specific signaling exchanged and/or specific functionality performed in a specific order, some of the signaling/functionality may be omitted or some of the signaling/functionality may be combined, sub-divided, or reordered without departing from the scope of the claims. Thus, unless specifically indicated, the order and grouping of the signaling/functionality depicted is not a limitation of other embodiments that may lie within the scope of the claims.
Simplicity and clarity in both illustration and description are sought to effectively enable a person of skill in the art to make, use, and best practice the present invention in view of what is already known in the art. One of skill in the art will appreciate that various modifications and changes may be made to the specific embodiments described below without departing from the spirit and scope of the present invention. Thus, the specification and drawings are to be regarded as illustrative and exemplary rather than restrictive or all-encompassing, and all such modifications to the specific embodiments described below are intended to be included within the scope of the present invention.